I too have experianced this, but I run the AGCEEP-MyMap variant.
1) there are some decades (and much of the 16th C.) where many events are checking every day (or every few days) for their triggers.
2) MyMap is large, and has alot of nations. As their tech and knolwedge of their neighbours goes up for nations they start sending out merchants, and relations with other nations have to be calculated. It gets more complicated the more nations there are, and it isnt linear. Furthermore, with more provinces there are also more buildings being constructed, more recruitment of troops - somewhere. Theres also weather, revolt risk and taxes to calculate for all of them. And of course, graphics/sprites for the lot.
3) celeron 2.0 + 512 RAM will run it, but slowly. It bites, I know - I have/had a similar system.